Saturday, May 1, 2010

Are Overweight Women at Risk of Fibromyalgia?

In The News Overweight and obese women — especially those who do not exercise — are at higher risk for developing fibromyalgia syndrome, according to new research in the May 2010 issue of Arthritis Care & Research. However, this may be more a case of flawed research than actual fact.

Researchers at the Norwegian University of Science and Technology, Trondheim, Norway, retrospectively examined survey data collected during 1984 to 1986 from a large longitudinal sampling of females (n=15,990) without fibromyalgia syndrome or physical impairments at baseline [Mork et al. 2010]. Physical exercise (frequency, duration, and intensity) and body mass index (BMI) were recorded at baseline and these data were subsequently used to assess the risk of having fibromyalgia at an 11-year followup during 1995 to 1997.

At followup, 380 cases of incident fibromyalgia were reported (2.4% of total sample), and the researchers noted a weak dose-response association between level of physical exercise and risk of fibromyalgia (p = 0.13 for trend); women who had reported the highest exercise level had a lower relative risk (RR) of 0.77 (95% confidence interval 0.55-1.07) for developing fibromyalgia. BMI was an independent risk factor for fibromyalgia (p < 0.001), and overweight or obese women (BMI 25.0 kg/m2 or greater) had a 60% to 70% higher risk compared with women with normal weight (BMI 18.5-24.9 kg/m2). BMI takes height and weight into account to measure body fat (BMI = weight in kg divided by height in meters squared: >25 is considered overweight, >30 is obese, and >40 is severely obese).

Overweight or obese women who exercised 1 hour per week had a fibromyalgia RR=1.72 (95% CI: 1.07-2.76) compared with normal-weight women with a similar activity level, whereas the risk was >2-fold higher for overweight or obese women who were either inactive (RR 2.09; 95% CI: 1.36-3.21) or exercised less than 1 hour per week (RR 2.19; 95% CI: 1.39-3.46). Consequently, the researchers conclude that being overweight or obese was significantly associated with an increased risk of developing fibromyalgia, especially among women who also reported low levels of physical exercise.

COMMENTS & CAVEATS: According to the National Fibromyalgia Association, fibromyalgia is one of the most common chronic pain conditions, affecting an estimated 3% to 6% of the world population and up to 90% of those afflicted are women. Therefore, the 2.4% incidence rate of fibromyalgia found among women during 11 years in this study population is not unexpected and, in fact, seems rather low. Exactly how being overweight or obese could increase the risk of developing fibromyalgia is not understood but, according to one of the many news reports on the study [Mann 2010], “some research suggests that increased levels of certain inflammatory proteins may play a role in both fibromyalgia and obesity.” This presumption seems both vague and speculative.

Furthermore, we have cautioned previously about the inherent dangers and misrepresentations of “data mining” [here]; that is, taking a body of data (quite old data in the case of this present study) and retrospectively searching for what appear to be secondary, statistically meaningful associations. Had there been adequate data available for analysis, the Norwegian researchers might also have found additional significant associations of fibromyalgia and BMI with depressed mood, diet, arthritis, childhood trauma, or a host of other factors that may or may not be relevant. Their own data found an association between physical exercise and fibromyalgia that they conceded was a “weak trend” but is actually of no significance (p > 0.05, with Relative Risk CI crossing 1.0).

While the study may be construed as highlighting a connection between exercise, body weight, and well-being, this is only common sense. Certainly, regular physical exercise and maintenance of normal weight would be recommended for all patients as a personal health measure, but we are reluctant to conclude from this study that doing so might significantly reduce a woman’s chances of developing fibromyalgia syndrome.

> Mann D. Obesity May Raise Risk of Fibromyalgia. WebMD Health News. 2010(Apr 29) [
available here].
> Mork PJ, Vasseljen O, Nilsen TIL. Association between physical exercise, body mass index, and risk of fibromyalgia: Longitudinal data from the Norwegian Nord-Trøndelag Health Study. Arthritis Care & Research. 2010;62(5):611-617 [
abstract here].